ABSTRACT Itch is defined as an unpleasant sensation that evokes a desire to scratch. In contrast to acute itch that is transient, chronic itch is a persistent, debilitating condition, which has few treatment options. Although studies have identified a number of essential molecules that transduce acute itch signals, we are only now beginning to uncover the cellular and molecular players that drive chronic itch in primary afferent neurons and spinal neurons. Advancing treatment technology for itch will require discovery of the molecular players that transduce itch sensations, which can ultimately serve as targets for therapeutics. The goal of this proposal is to identify novel genes and biomolecules that underlie itch, focusing on the signaling mechanisms in primary afferent neurons, immune cells and spinal cord modulatory interneurons. Somatosensory afferents are activated by itch-producing compounds that are released by a variety of cells in the skin, including keratinocytes. Pruritogens trigger somatosensory neuron activation by binding to G-protein coupled receptors and opening transduction channels that depolarize the nerve terminal and promote action potential firing; these neurons then signal to itch- specific neurons in the spinal cord. While recent studies have begun to delineate the basic characteristics of the itch circuit, the molecular mechanisms underlying itch have yet to be identified: the receptors, transduction channels and downstream signaling factors are largely unknown. This grant proposal describes the development of new genetic approaches to meet this challenge. We are two biologists with experience and expertise in sensory neurobiology, genetics, and genomics who seek to identify the genes that drive itch behaviors. We will analyze the natural variation between genetically distinct mouse strains in itch-evoked behaviors and identify sequence and gene expression differences that underlie such phenotypic change. In contrast to traditional genetic screening approaches, which are not easily applicable to live-animal phenotypes in the mouse, the genetic mapping paradigm has the potential to survey a genome's worth of genetic perturbations and uncover novel determinants of itch. Intolerable itch accompanies numerous skin and nervous system disorders, and in most cases, is insensitive to antihistamine treatment. Identification of candidate itch factors will provide new targets for development of drugs and therapies to treat intractable itch.